The relationship of drug use to the subsequent development of cancer will be studied in two large populations among subscribers of the Kaiser Foundation Health Plan in Northern California. One population will consist of 234,000 persons who utilized the Kaiser-Permanente San Francisco clinic from 1969-1973, among whom 149,000 have computer-stored records documenting receipt of prescription drugs from the facility's pharmacy. The other will consist of an estimated 25,000 persons who will complete a life-term drug history questionnaire during multiphasic health checkups at the Kaiser-Permanente facilities in Oakland, Sacramento, San Franciso and Santa Clara. Follow-up for cancer development will rely primarily on diagnostic data available within Kaiser-Permanente plus the Cancer Incidence System of the California State Health Department's Resource for Cancer Epidemiology. As an initial screen for possible drug effects, incidence rates for various cancers will be compared among users and non-users of the drugs studied. Further ad hoc studies will be carried out on selected drug-cancer associations to determine the likelihood of a causual relationship. In this way we propose to utilize a unique resource having both drug usage information and good follow-up, to detect possible carcinogenic effects or affirm the relative safety of commonly used medicinal drugs.